Tunable lasers for use in optical communications systems, particularly in connection with wavelength division multiplex (WDM) telecommunication systems, are known. A known tunable system comprises stacks of single wavelength distributed Bragg reflectors (DBR) lasers, which can be individually selected; or tuned over a narrow range, or by a wide range tunable laser that can be electronically driven to provide the wavelength required. Limited tuning range tunable lasers that rely upon thermal effects for tuning are also known.
Atypical optical fibre telecommunications band is the 1550 nm C-band, located in the infra-red spectrum with International Telecommunication Union (YrU) 200, 100 or 50 GHz channel spacing (the so-called ITU Grid) spread between 191 THz and 197 THz.
U.S. Pat. No. 4,896,325 discloses a wavelength tunable laser having sampled gratings at the front and rear of its gain region. The laser described in that specification relies on the use of two different gratings which produce slightly different reflection combs in the front and rear gratings. These provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Co-incidence of a maximum from each of the front and rear gratings is referred to as a supermode. To switch the device between supermodes requires a small incremental electrical current into one of the gatings to cause a different pair of maxima to coincide in the manner of a vernier. By applying electrical currents to the two gratings so that the corresponding maxima track, continuous tuning within a supermode can be achieved.
Alternative forms of electrically tunable comb reflection gratings are commonly known as superstructure gratings such as described in “Broad-Range Wavelength-Tunable Superstructure Grating (SSG) DBR Lasers”, Y Thomori et al, IEEE Photonics Technical Letters, Volume 5, No. 6, Jul. 1993.
The applicant's co-pending UK application 0106790.9 discloses a tunable laser having a gain section, a phase section and a segmented Bragg grating reflector comprising a series of grating units, at least two of which have a different pitch, wherein an electrical current is applicable to the grating having a longer pitch such that the wavelength of the longer pitch grating can be tuned to the wavelength of the shorter pitch grating.
Another tunable laser is disclosed in the applicant's copending UK application 0118412.6 in which there is disclosed a tunable laser including a gain section bounded at one end by a first reflector in the form of a distributed Bragg grating reflector producing a first comb of reflective peaks and on the other end by a second reflector in the form of a segmented distributed Bragg grating reflector with each segment capable of producing a peak corresponding to one of the peaks in the first comb, some at least of the segments in the segmented distributed Bragg grating reflector being capable of modification so as to reflect light at the wavelength of another peak so as to form a reinforced peak, so that the laser is capable of lasing at the wavelength of the reinforced peaks.
The segmented distributed Bragg grating reflector comprises a plurality of segments, typically eight or nine, each comprising gratings, the different segments having a grating which may be each of different pitch and their preferred organization being the longest pitch is at the facet end of the reflector, and the shortest pitch is closest to the gain section, with progressive pitch size change therebetween.
The segments are modified to change the reflecting wavelength by altering the refractive index of the material of which the segments are formed, and the refractive index may be varied by passing an electrical current through the segments. Each segment has an associated electrode to permit the passage of current through the segment arid each electrode is actuable independently of the other electrodes.
The common method of controlling multi-section lasers is to provide one digitally controlled current source per electrode and it is common to implement each current source digitally using a digital-to-analogue-converter (DAC). However for a tunable laser having multiple sections and a plurality of independently actuable grating segments, to provide one digitally controlled DAC per section or segment has major disadvantages in circuit size, control complexity and consequential costs.
It is an object of the present invention to provide a suitable control for a tunable laser having a segmented distributed Bragg grating reflector, which ameliorates these problems.